Method for forming a nonfull aperture luneberg lens with a graded index core and a homogenous cladding

ABSTRACT

Disclosed is a nonfull aperture Luneburg-type lens for correction of an adjacent light source. The lens includes a core having a circular cross-section and a graded refractive index, and a cladding enclosing the core. The cladding has a circular cross-section and a homogenous refractive index. Also disclosed is a method for forming the nonfull aperture Luneburg-type lens with a graded index core and a homogenous cladding, and a high numerical aperture laser diode assembly including the nonfull aperture Luneburg-type lens.

FIELD OF THE INVENTION

The present invention relates to a method for forming a nonfull apertureLuneburg-type lens with a graded index core and a homogenous cladding,to a nonfull aperture Luneburg lens with a graded index core and ahomogenous cladding, and to a high numerical aperture laser diodeassembly.

BACKGROUND OF THE INVENTION

It is well known that laser diodes alone produce a beam that isdivergent and astigmatic. To get better performances from a laser diode,lenses can be placed in front of the beam emitted by the laser diode, toimprove its performances.

Different types of lenses can be used to correct the divergence,symmetry and astigmatism of laser diodes. Because laser diodes have anelongated rectangular aperture through which the beam is emitted, themost widely used type of lens is the cylindrical lenses.

Existing cylindrical lenses used for correcting laser diodes are made ofa homogeneous medium, and have a cross-section either circular ornoncircular. The cylindrical lenses of circular cross-section are easyto form, but they have poor optical performance when used at highnumerical aperture, due to the large spherical aberrations. Thecylindrical lenses of noncircular cross-section are capable of producinga better quality beam, but they are more difficult to produce since theyrequire precision grinding of a relatively complex surface and precisecentering of the two surfaces forming the lens. In use, the noncircularcylindrical lenses require precise positioning of the lens relative tothe laser diode to obtain good results.

There are different types of lenses that have been termed as Luneburglenses. The common threads for all of them are: the spherical symmetry(ball shape) or at least circular cross-section, the aberration-freeimaging, except for chromatic aberration and field curvature, and thedesign principles where the graded index profile is calculated frompre-selected image and object positions. The main problem associatedwith the design of the Luneburg graded index lenses is to find thedesign whose refractive index distribution can be realized with theselected technology. With Luneburg-type cylindrical lens it is possibleto preserve the circular shape of the lens without introducingaberrations.

Known in the art are U.S. Pat. Nos. 5,080,706 and 5,155,631 (Snyder etal) which describe methods for fabrication of cylindrical microlenses ofselected shape. These methods consist in first shaping a glass preforminto a desired shape. Then, the preform is heated to the minimum drawingtemperature and a fiber is drawn from it. The cross-sectional shape ofthe fiber is cut into sections of desired lengths. Finally, the fiber iscut into sections of desired lengths.

Also known in the art, is U.S. Pat. No. 5,181,224 (Snyder) whichdescribes microlenses. This patent provides several microlensconfigurations for various types of optical corrections.

Another patent known in the art is U.S. Pat. No. 5,081,639 (Snyder etal), which describes a laser diode assembly including a cylindricallens. This assembly comprises a laser diode and a cylindrical microlenswhose cross-section is different from circular.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide a methodof forming a nonfull aperture Luneburg lens with graded index core andhomogenous cladding that is simple and that does not need precisegrinding and precise centering. It is also another object of the presentinvention to provide a nonfull aperture Luneburg lens with graded indexcore and homogeneous cladding that requires less stringent positioningrelative to an adjacent source while offering good beam correction. Itis also another object of the present invention to provide a highnumerical aperture laser diode assembly that is simple to build, notexpensive and that offers good performance.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a method forforming a nonfull aperture Luneburg-type lens with graded index core andhomogenous cladding for correction of an adjacent light source, the lenshaving a normalized external radius, a core of radius a, an objectdistance s₁ and an image distance s₂, the method comprising the stepsof:

a) selecting a cladding having a refractive index N, an internal radiusand an external radius;

b) calculating a refractive index profile n(r), where r is the distancefrom the center of the core, and corresponding to said a, s₁, s₂ and N;

c) making a preform by introducing graded optical material inside ofsaid cladding from step (a) to obtain the refractive index profilecalculated in said step (b) and by collapsing said cladding and saidgraded optical material; and

d) after said step (c), drawing the preform into a nonfull Luneburg-typelens having desired external radius.

Preferably, the refractive index profile is calculated with: ##EQU1##where "*" means multiplication.

It is another object of the invention to provide a nonfull apertureLuneburg lens for optical correction of an adjacent light source, theLuneburg lens comprising:

a core having a circular cross-section and a graded refractive indexdistribution; and

a cladding enclosing the core, the cladding having a circularcross-section and a homogenous refractive index.

Preferably, the nonfull aperture Luneburg lens has a cylindrical shapeand is drawn from a drawn glass preform.

Also, another object of the present invention is to provide a highnumerical aperture laser diode assembly comprising:

a laser diode source having an elongated rectangular aperture foremitting a laser beam through the elongated rectangular aperture;

a Luneburg cylindrical lens parallel to the elongated rectangularaperture and set in front of the laser beam for optical correctionthereof, the Luneburg lens comprising:

a core having a circular cross-section and a graded refractive indexdistribution; and

a cladding enclosing the core, the cladding having a circularcross-section and a homogeneous refractive index.

Preferably, the Luneburg lens is drawn from a glass preform.

A non restrictive description of a preferred embodiment will now begiven with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a method for forming a nonfull apertureLuneburg lens with a graded index core and a homogeneous claddingaccording to the invention;

FIG. 2 is a perspective view showing a nonfull aperture Luneburg lenswith a graded index core and a homogeneous cladding according to theinvention; and

FIG. 3 is a side elevational view of a high numerical aperture laserdiode assembly according to the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

Several characteristics are used to describe a lens. More particularly,to describe lenses that are made from a glass preform, thecharacteristics are: the size of the core, which is described by itsradius, the size of the cladding, described by its external radius, theobject distance, and the image distance. Here, the radius of the core iscalled a, the external radius of the cladding is unitary, the objectdistance is called s₁ and the image distance is called s₂. These valuesare scalable to whatever the external size of the cladding is. Moreover,the expression "optically correcting" refers to aberration correctionbut does not include chromatic aberration and field curvature.

Referring now to FIG. 1, there is shown a method 1 for forming a nonfullaperture Luneburg lens with a graded index core and a homogeneouscladding. The first step of the method consists in selecting 3 acladding 19, characterized by its external and internal radii and by arefractive index N.

The next step of the method consists in calculating 5 a refractive indexprofile n(r), where r is the distance from the center of the core. Thatrefractive index profile takes into account the desired characteristicsof the lens to be made. These characteristics are the desired coreradius, the desired object distance and the desired image distance for aunit cladding radius. The refractive index profile is calculated withformula (1). ##EQU2##

In this formula, the expression ρ relates to the refractive index of thecore of the lens, such as described in equation (2):

    ρ=ρ(r)=n(r)*r, where 0≦r≦            (2)

a and where "*" represents multiplication;

This refractive index of the core ρ is calculated with equation (3).##EQU3##

In equations (1), (2) and (4), P_(a) and P₁ are determined by:

    P.sub.a =N*a;                                              (6)

    P.sub.1 =N;                                                (7)

where P_(a) and P₁ are particular cases of P(r) which is a speciallyselected function for the cladding, such as defined in the nextequation:

    P=P(r)=N*r, where a≦r≦1;                     (8)

The expression Ω(ρ,s,P) of equation (1) can be generally expressed bythe following equation: ##EQU4##

Thus, the Ω expressions in equation (1) represent the following:##EQU5##

The following step consists in making a preform by introducing 7 gradedoptical material inside of the chosen cladding and by collapsing thecladding and the graded optical material, to obtain the refractive indexprofile calculated in the previous step.

The graded optical material may be deposited directly inside the tube bymeans of a modified chemical vapour deposition process.

Another way to make a preform consists of using an ion-exchange process.That process is used to modify the refractive index profile of a glassrod to correspond to the refractive index profile calculated in step(b), and to introduce the glass rod inside of the cladding of step (a),the glass rod becoming the core of the preform.

Step (c) may also further consist of verifying whether the refractiveindex profile of said preform is substantially equal to the refractiveindex profile calculated in step (b), and if necessary, repeating steps(a), (b) and (c) and slightly changing deposition parameters until therefractive index profile of the preform is substantially equal to therefractive index profile calculated in step (b).

Finally, the last step consists of drawing 9 the preform into a Luneburglens having desired radius. The method 1 may also comprise one last stepwhich consists of cutting 11 the drawn preform to a predeterminedlength.

This method is thus simple and it does not need precise grinding norprecise centering. The circular shape of the lens makes it easilyscalable for a wide range of focal lengths.

Referring now to FIGS. 2 and 3, there is shown a nonfull apertureLuneburg lens 13 for optical correction of an adjacent light source 15,such as a laser diode. That lens 13 has a cylindrical shape. It has acore 17 of circular cross-section and graded refractive indexdistribution. The lens 13 has a cladding 19 enclosing the core 17. Thecladding 19 has a circular cross-section and a homogeneous refractiveindex. The refractive index distribution inside the core 17 corrects theaberrations of the cladding 19 but only for the light rays that alsopass through the core 17, hence the name nonfull aperture lens. Thatlens 13 could be made, for example, from a drawn glass preform having anouter portion made of fused silica.

The lens has to be placed in front of the aperture (not shown) of thelight source 15 to optically correct its beam. Its circular form andvery good aberration correction at high numerical apertures makes thislens 13 less sensitive to positioning errors.

Referring now to FIG. 3, there is shown a high numerical aperture laserdiode assembly. This assembly comprises a laser diode source 15 and anonfull aperture Luneburg cylindrical lens 13. The laser diode source 15has an elongated rectangular aperture (not shown) for emitting a laserbeam 21 through that aperture. The Luneburg cylindrical lens 13 isplaced parallel to the elongated rectangular aperture and set in frontof the laser beam 21 for optically correcting that beam.

The Luneburg lens 13 has a core 17 and a cladding 19. The core 17 has acircular cross-section and a graded refractive index distribution, andthe cladding 19 encloses the core 17. The cladding 19 has a circularcross-section and a homogeneous refractive index. The Luneburg lens 13may be made from a drawn glass preform having a cladding made of fusedsilica.

Although a preferred embodiment of the invention has been described indetail herein and illustrated in the accompanying drawings, it is to beunderstood that the invention is not limited to this precise embodimentand that various changes .and modifications may be effected thereinwithout departing from the scope or spirit of the invention.

What is claimed is:
 1. A method for forming a nonfull aperture Luneburg lens with a graded index core and a homogeneous cladding for correcting an adjacent light source, the lens having a normalized external radius, a core radius a, an object distance s₁ and an image distance s₂, the method comprising the steps of:a) selecting the cladding, the cladding having a refractive index N, an internal radius and an external radius; b) calculating a refractive index profile n(r), where r is the distance from the center of the core, the refractive index profile being calculated using the equation: ##EQU6## c) making a preform by introducing an optical material inside of said cladding selected in step (a) for obtaining the refractive index profile calculated in said step (b) and by collapsing said cladding and said introduced optical material; and d) after said step (c), drawing the preform into a nonfull aperture Luneburg lens having a normalized external radius.
 2. The method of claim 1, wherein in said step (c), the optical material is introduced inside of said cladding by means of a modified chemical vapour deposition process.
 3. The method of claim 1, further comprising a step (e) wherein after step (d), the drawn preform is cut to a predetermined length. 